A 67-year-old woman presented with a swelling on both sides of the neck. Biopsy of an enlarged cervical lymph node on the left side and flow cytometric analysis revealed CD56-positive CD4(+)CD8(+) abnormal NK/T cells. A Southern blot analysis of the cervical lymph node biopsy specimen showed a human T-cell leukemia virus type 1 provirus DNA monoclonal band. Based on these findings, the patient was diagnosed with CD56-positive adult T-cell leukemia/lymphoma. After five cycles of cyclophosphamide, doxorubicin, vincristine, and prednisone therapy, the general condition of the patient gradually declined, indicating resistance to treatment, and approximately 9 months after the onset of symptoms, the patient died. CD56 is recognized as an unfavorable prognostic marker in cases of acute myeloid leukemia with t(8;21), acute promyelocytic leukemia, and anaplastic large cell lymphoma. Only eight cases of CD56-positive adult T-cell leukemia/lymphoma have been reported so far in the literature. Most of these cases were in the advanced stage at diagnosis and had poor outcomes. It appears that the correlation between CD56 expression and outcomes in patients with adult T-cell leukemia/lymphoma should be clarified by investigating a larger number of cases in the future.